1. Field of the Invention
The present invention relates generally to the field of nuclear reactor primary system decontamination and, more particularly, to an improved system for removing suspended and dissolved solids from nuclear reactor primary system fluids after the primary system has been subjected to a chemical decontamination process.
2. Description of the Prior Art
The problem of excessive personnel exposures caused by high background radiation levels in a nuclear reactor primary system, such as in a pressurized water reactor (PWR) system, and the resultant economic cost of requiring personnel rotation to minimize individual exposure is significant at many nuclear plants. These background levels are principally due to the build-up of corrosion products in certain areas of the plant. The build-up of corrosion products exposes workers to high radiation levels during routine maintenance and refueling outages.
As a nuclear power plant operates, the surfaces in the core and other portions of the primary system corrode. Corrosion products, referred to as "crud", are activated by transport of the corroded material to the core region by the reactor coolant system. Subsequent release of the activated crud and redeposition elsewhere in the system produces radiation fields in piping and components throughout the primary system, thus increasing radiation levels throughout the plant. The activity of the corrosion product deposits is predominantly due to cobalt 58 and cobalt 60. It is estimated that between 80% and 90% of personnel radiation exposure can be attributed to these elements.
One way of controlling worker exposure, and of dealing with this problematic situation, is to periodically decontaminate the nuclear steam supply system using chemicals, thereby removing a significant fraction of the corrosion product oxide films. Prior techniques have done very little to decontaminate the primary system as a whole, typically focusing only on the heat exchanger (steam generator) channel heads.
Two different chemical processes, referred to as LOMI (developed in England under a joint program by EPRI and the Central Electricity Generating Board) and CAN-DEREM (developed by Atomic Energy of Canada, Ltd.), have been used for small scale decontamination in the past. These processes are multi-step operations, in which various chemicals are injected, recirculated, and then removed by ion exchange. Although the chemicals are designed to dissolve the corrosion products, some particulates are also generated. Another method of chemical decontamination which focuses on the chemistry of decontamination, is disclosed in U.K. Patent Application No. GB 2 085 215 A (Bradbury et al. ) . However, this patent application discloses little of the methodology to be used in applying that chemistry to full scale nuclear reactor primary system decontamination.
While these chemical processes had typically been used only on a localized basis, the use of these chemical processes has now been considered for application on a large scale, full system chemical decontamination. Such an application is disclosed generally in U.S. Pat. No. 5,089,216, assigned to the assignee of the present invention and entitled "System for Chemical Decontamination of Nuclear Reactor Primary Systems".
The estimated collective radiation dose savings over a 10-year period following nuclear reactor primary system decontamination is on the order of 3,500-4,500 man rem, depending upon whether or not the fuel is removed during decontamination. At any reasonable assigning of cost per man rem, the savings resulting from reduced dosage levels will be in the tens of millions of dollars.
As a result of the recognition that full nuclear reactor primary system chemical decontamination is indeed possible from a technical standpoint and, more importantly, advantageous in order to increase collective radiation dose savings, systems have been suggested for effective and economic removal of dissolved and particulated corrosion products generated by the application of these known chemical decontamination techniques. One such system is disclosed in U.S. Pat. No 5,089,217, assigned to the assignee of the present invention and entitled "Clean-Up Sub-System for Chemical Decontamination of Nuclear Reactor Primary Systems". In this sub-system, chemically-processed fluids containing suspended and dissolved solids are directed first through a back-flushable filter and, thereafter, through one or more cartridge filters. After this initial filtering of suspended sol ids, these fluids are directed to one or more banks of demineralizers for removal of dissolved solids, followed by additional cartridge filtering to remove any resin fines carried out of the demineralizer banks. After final filtering, the fluids are returned to the primary system. Although this clean-up sub-system adequately removes dissolved and suspended solids from nuclear reactor primary system fluids, the system as designed requires that the resin beds within each of the demineralizers be replaced at least once during the chemical decontamination clean-up process. Although the replacement of demineralizer resin beds may be accomplished utilizing known demineralizer back-flushing and refilling techniques, replacement of these demineralizer resin beds during the clean-up process requires additional operator attention and increases the cost of the clean-up equipment (i.e., storage tanks, pumps, valves, piping and electrical control devices) due to the additional storage and handling of the spent resin which must be accomplished without delay the clean-up process itself.
Consequently, there is a need for an improved clean-up system which operates effectively and economically to remove dissolved and particulated corrosion products from primary system fluids generated by the application of known chemical decontamination techniques.